1. Field of the Invention
The present invention relates to a machine tool that grips a rod-shaped workpiece at a leading edge of a main spindle and that subjects the workpiece to machining, such as cutting or boring, and more particularly to a machine tool made up of a lathe of main spindle movable type that moves a main spindle along an axis during machining.
2. Background Art
In relation to a machine tool of this type, a configuration as described in; for example, JP-UM-B-3-40488, has hitherto been proposed as a lathe of main spindle movable type that subjects an elongated rod-shaped workpiece to machining.
In the related-art configuration described in JP-UM-B-3-40488, a guide bush for supporting forward of the main spindle the leading edge of the workpiece in an insertable manner can be removably attached. When the guide bush is in an attached state, the workpiece gripped by the main spindle is also supported by the guide bush and subjected to machining, such as cutting, forward of the guide bush by means of a cutting tool on the tool post. In this case, the guide bush receives a load that acts on a workpiece during machining in a direction orthogonal to the axial direction of the workpiece. Therefore, even when the workpiece is elongated and easily deflectable, the workpiece can be machined with high precision.
In the meantime, in the related-art configuration of JP-UM-B-3-40488, when the guide bush is removed, a short workpiece can be machined. In relation to a lathe of main spindle movable type arranged so as to be able to machine a short workpiece, a configuration such as those described in connection with JP-A-2006-326732 and JP-UM-B-7-19694 has been proposed.
In the related-art configurations described in JP-A-2006-326732 and JP-UM-B-7-19694, the guide bush is not provided.
In a configuration described in JP-A-2006-326732, a through hole for causing the main spindle to pass in a non-contacting state is formed in the tool post support. In association with advancement of the headstock, the leading edge of the main spindle penetrates through the through hole, to thus project forward of the tool post support, and the workpiece is machined by means of the cutting tool on the tool post. Further, in a configuration described in JP-UM-B-7-19694, the headstock is supported in a penetrating manner on an upper frame so as to become movable in the axial direction of the main spindle by way of a horizontal sliding guide surface and a vertical sliding guide surface arranged in the shape of a cross. The workpiece is machined forward of the upper frame by means of the cutting tool on the tool post.
Incidentally, the related-art configurations present the following drawbacks.
First, in the related-art configuration described in JP-UM-B-3-40488, in a state where the guide bush is attached forward of the main spindle, space including the length of the guide bush is formed between the main spindle and the guide bush, and a remaining material of a workpiece whose length corresponds to the space arises. Therefore, when the guide bush is in an attached state, the workpiece cannot be utilized effectively. In addition, in the case of a short workpiece, the guide bush interferes with the workpiece, thereby posing difficulty in machining the short workpiece.
In the related-art configuration of JP-UM-B-3-40488 in which the guide bush is removed and the related-art configurations of JP-A-2006-326732 and JP-UM-B-7-19694, no guide bush is present, and hence a problem such as that mentioned above does not occur.
However, in the related-art configuration of JP-UM-B-3-40488 in which the guide bush is removed, a guide sleeve is attached onto a guide bush support bed in lieu of the guide bush on the support bet, and a cylindrical sleeve attached a leading edge of a main spindle is slidingly guided along an internal peripheral surface of the guide sleeve constituting a sliding bearing. Therefore, in this state, the main spindle is substantially extended forwardly by an amount corresponding to the length of the cylindrical sleeve. In this state, there is required guide rigidity of the order of magnitude which enables only a base end of the main spindle to support the main spindle. Therefore, in order to firmly guide the leading edge of the main spindle, by means of a sliding bearing, to such an extent that load imposed during machining can be born, the precision of a guide section on the leading-edge side of the main spindle with respect to a guide section on the base-end side of the main spindle must be enhanced extremely high. In reality, acquisition of such precision is impossible. Even if such precision can be achieved, very high cost will be incurred. In order to solve the problem, the rigidity of the guide on the leading-edge side of the main spindle must be weakened considerably. Therefore, if an attempt is made to actually provide the configuration described in JP-UM-B-3-40488, there is no alternative way but to weaken the rigidity of the guide to such an extent that the load imposed during machining cannot be born by the leading-edge side of the main spindle. For this reason, the base-end side of the main spindle chiefly bears load in an overhanging state as described in connection with JP-A-2006-326732, so that the rigidity of the main spindle resultantly decreases and that machining precision is reduced.
In the related-art configuration described in JP-A-2006-326732, a guide member for guiding and supporting the headstock so as to be movable in the axial direction of the main spindle is disposed at the rear of the tool post support so as to avoid interference with the tool post support. The main spindle and the leading-edge portion of a support sleeve supporting the main spindle are disposed in a cantilever fashion while greatly overhanging forwardly from the guide member. Therefore, difficulty is encountered in imparting high support rigidity to the main spindle during machining, which in turn reduces machining precision.
Moreover, in the related-art configuration described in JP-UM-B-7-19694, the upper frame and the cutting tool are in close proximity to each other, and hence machining with high rigidity is possible. However, the guide configuration of the headstock with respect to the upper frame is made up of a horizontal sliding guide surface and a vertical sliding guide surface that are, on the whole, arranged in the shape of across. Therefore, the configuration of the guide surfaces is complicate. Further, since the number of guide surfaces is large, there is a potential risk of a warp, and machining guide surfaces involves consumption of much effort, as well.